Biomedical subjects

O Betancourt

Publications and source records attributed to O Betancourt.

7 recordsLinked to original sources

PubMed2006

Decrease of soil fertility and release of mercury following deforestation in the Andean Amazon, Napo River Valley, Ecuador.

Soil erosion and degradation provoked by deforestation in the Amazon is a global concern, and recent studies propose a link between deforestation, soil erosion and the leaching of naturally occurring mercury (Hg). In the Ecuadorian Amazon, elevated deforestation rates and the proximity of volcanoes could play an important role in soil fertility and soil Hg levels. The goal of this study is to evaluate the impacts of deforestation on Andisol and Inceptisol fertility and Hg levels in the Napo River Valley, Ecuador. Results show a significant decrease in surface soil organic matter (-15% to -70% of C and N) and exchangeable cations (-25% to -60%) in deforested plots. Hg concentrations at the surface (0-5 cm), higher in Andisols (225 ng/g average) than in Inceptisols (95 ng/g average), show a decrease of up to 60% following deforestation. Soil erosion exposes the mineral horizon, a layer with a higher Hg burden, to the elements thus provoking and accelerating Hg leaching. These results suggest that deforestation and the associated Hg leaching could contribute to the fish Hg contamination measured in the Napo River watershed.

Agriculture↗

PubMed1988

Nonlinear saturation of ballooning modes in tokamaks and stellarators.

The spectral code BETAS computes plasma equilibrium in a toroidal magnetic field B = [unk]s x [unk]Psi with remarkable accuracy because the finite difference scheme employed in the radial direction allows for discontinuities of the flux function Psi across the nested surfaces s = const. Instability of higher modes in stellarators like the Heliotron E can be detected in roughly an hour on the best supercomputers by calculating bifurcated equilibria that are defined over just one field period. The method has been validated by comparing results about nonlinear saturation of ballooning modes in tokamaks with numerical data from the PEST code.

Journal Article↗

PubMed1986

Monte Carlo estimation of the electric field in stellarators.

The BETA computer codes have been developed to study ideal magnetohydrodynamic equilibrium and stability of stellarators and to calculate neoclassical transport for electrons as well as ions by the Monte Carlo method. In this paper a numerical procedure is presented to select resonant terms in the electric potential so that the distribution functions and confinement times of the ions and electrons become indistinguishable.

Journal Article↗

PubMed1981

Beta limits for torsatrons.

An ideal magnetohydrodynamic equilibrium and stability code is used to study ballooning modes in torsatrons. The most dangerous modes turn out to be those with low poloidal and toroidal wave numbers. Beta limits for equilibrium and stability are determined for an [unk] = 2 ultimate torsatron with large [unk] = 1 and [unk] = 3 sidebands.

Journal Article↗

PubMed1977

A method of computational magnetohydrodynamics defining stable Scyllac equilibria.

A computer code has been developed for the numerical calculation of sharp boundary equilibria of a toroidal plasma with diffuse pressure profile. This generalizes earlier work that was done separately on the sharp boundary and diffuse models, and it allows for large amplitude distortions of the plasma in three-dimensional space. By running the code, equilibria that are stable to the so-called m = 1, k = 0 mode have been found for Scyllac, which is a high beta toroidal confinement device of very large aspect ratio.

Journal Article↗

PubMed1976

Equilibrium and stability code for a diffuse plasma.

A computer code to investigate the equilibrium and stability of a diffuse plasma in three dimensions is described that generalizes earlier work on a sharp free boundary model. Toroidal equilibria of a plasma are determined by considering paths of steepest descent associated with a new version of the variational principle of magnetohydrodynamics that involves mapping a fixed coordinate domain onto the plasma. A discrete approximation of the potential energy is written down following the finite element method, and the resulting expression is minimized with respect to the values of the mapping at points of a rectangular grid. If a relative minimum of the discrete analogue of the energy is attained, the corresponding equilibrium is considered to be stable.

Journal Article↗

PubMed1975

Computer simulation of the toroidal equilibrium and stability of a plasma in three dimensions.

A computer program has been written to solve the equations for sharp boundary magnetohydrodynamic equilibrium of a toroidal plasma in three dimensions without restriction to axial symmetry. The numerical method is based on a variational principle that indicates whether the equilibria obtained are stable. Applications have been made to Tokamak, Stellarator, and Scyllac configurations.

Journal Article↗